A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) of a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Although specific reference may be made herein to the use of lithographic apparatuses in the manufacture of ICs, it should be understood that the lithographic apparatuses described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in a track tool that typically applies a layer of resist to a substrate and develops the exposed resist, a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.
Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example, imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate, whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist, leaving a pattern in it after the resist is cured.
The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5–20 nm), as well as particle beams, such as ion beams or electron beams.
The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.
The position of the substrate should be known accurately with respect to a projection system used to apply the desired pattern onto the substrate. The projection system focuses the radiation beam that is provided with a pattern by the patterning device onto the substrate. In order to achieve optimal results, first height measurements are performed for the surface of the substrate, generating height data. Such height data may include information about the morphology of the surface of the substrate. In case a lithographic apparatus having two or more substrate tables (dual stage) is used, the height measurements may be performed on a first position (measurement position), while the exposure is done at a second position (exposure position). In that case, the height data obtained at the first position could be stored in a height map for later use during exposure at the second position. The height map consists of a set of data representing the height of the surface of the substrate with respect to a reference level at different positions on the substrate. However, the height measurements may also be performed on-the-fly (during exposure) and used in real-time during the exposure, for instance in case a single stage machine is used.
The height data is used to position the substrate during exposure as accurate as possible with respect to the projection system, taking into account the local and global shape of the substrate. Based on the height data, the relative position of the substrate with respect to the projection system can be adjusted for each target portion or even for different parts of a target portion.
Several level sensors are known that are arranged to obtain such height data. For instance, level sensors are known that optically measure the height of the substrate by projecting a radiation beam at an angle to the position on the substrate of which the height is to be measured. The level sensor comprises an array of sensors that each can detect a reflected beam. The reflected beam will hit one or more sensors depending on the height of the surface of the substrate at the position of the reflection. So, the height of the substrate at the position of the reflection can be determined.
The space between the projection lens and the substrate is rather small. In order to achieve higher numerical apertures, this space will become even smaller in future systems. Therefore, space that is available for the radiation beam and the reflected beam is becoming smaller and, as a result of this, fewer locations are available for positioning a level sensor.
Furthermore, optical level sensors may not be easily compatible in some instances with immersion techniques, in which part of the substrate is immersed. In lithographic machines using immersion techniques, part of the space between the projection lens and the substrate is filled with liquid, such as water. Devices, such as seals, are provided that keep the liquid in place. The radiation beam emitted by the optical level sensor that is positioned outside the seal should be guided through the seal to reach the surface of the substrate. This may be a somewhat difficult task to perform.
Also, radiation beams reflected by the top surface of the substrate may suffer from process dependencies. Process dependencies may be caused, for instance, by the fact that the radiation beam emitted by the level sensor is not only reflected by the top surface of the substrate, but also by sub-layers that are positioned under the top layer that may disturb the measurement. This disturbance is process dependent, i.e. it depends on the processes previously carried out on the substrate that determine the morphology of the sub-layers. Process dependencies may also be caused by phase shifts caused by reflections of metal surfaces.
The radiation beam may be reflected by a number of(sub-)layers and may combine to form one single reflected beam. The reflected beam can be thought of as a single beam reflecting once, somewhere below or above the surface of the substrate.
Other level sensors are known, such as capacitive or inductive level sensors. However, it will be understood that these level sensors may also suffer from process dependencies, since the electro-magnetic characteristics of the substrate may depend on the processes previously carried out on the substrate.
According to another type of level sensor, an air stream is used to determine the height of a substrate. This type of level sensor is called an air gauge and comprises an air outlet through which an air stream is directed. The outlet is positioned in the vicinity of the substrate and is directed perpendicular to the surface of the substrate. The air is thus directed to the surface of the substrate. Height differences of the substrate cause differences in the distance between the outlet of the air gauge and the substrate. The air gauge will therefore experience fluctuations in the resistance of the air flow that correspond to height fluctuations of the surface of the substrate. The resistance of the air flow is measured, from which height information is deduced. This could be done at different positions above the substrate to make an height map.
Such an air gauge is, however, a relatively slow level sensor, which may reduce the throughput of the system. Furthermore, the distance between the outlet and the surface of the substrate needs to be relatively small in order to perform accurate measurements. This requires accurate control mechanisms. Also, the air gauge can not be used in a single stage immersion lithographic apparatus.